首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   2878篇
  免费   212篇
  国内免费   2篇
  3092篇
  2021年   30篇
  2020年   20篇
  2018年   21篇
  2017年   23篇
  2016年   60篇
  2015年   83篇
  2014年   96篇
  2013年   140篇
  2012年   137篇
  2011年   144篇
  2010年   107篇
  2009年   107篇
  2008年   139篇
  2007年   140篇
  2006年   127篇
  2005年   112篇
  2004年   121篇
  2003年   104篇
  2002年   127篇
  2001年   54篇
  2000年   47篇
  1999年   37篇
  1998年   39篇
  1997年   38篇
  1996年   31篇
  1995年   35篇
  1994年   27篇
  1993年   22篇
  1992年   30篇
  1991年   22篇
  1990年   24篇
  1989年   35篇
  1988年   26篇
  1987年   22篇
  1985年   24篇
  1984年   29篇
  1983年   17篇
  1982年   27篇
  1981年   31篇
  1980年   15篇
  1979年   18篇
  1978年   17篇
  1977年   22篇
  1976年   20篇
  1973年   14篇
  1968年   15篇
  1963年   15篇
  1960年   16篇
  1951年   14篇
  1927年   13篇
排序方式: 共有3092条查询结果,搜索用时 0 毫秒
41.
42.
43.
Research needs a balance of risk‐taking in “breakthrough projects” and gradual progress. For building a sustainable knowledge base, it is indispensable to provide support for both. Subject Categories: Careers, Economics, Law & Politics, Science Policy & Publishing

Science is about venturing into the unknown to find unexpected insights and establish new knowledge. Increasingly, academic institutions and funding agencies such as the European Research Council (ERC) explicitly encourage and support scientists to foster risky and hopefully ground‐breaking research. Such incentives are important and have been greatly appreciated by the scientific community. However, the success of the ERC has had its downsides, as other actors in the funding ecosystem have adopted the ERC’s focus on “breakthrough science” and respective notions of scientific excellence. We argue that these tendencies are concerning since disruptive breakthrough innovation is not the only form of innovation in research. While continuous, gradual innovation is often taken for granted, it could become endangered in a research and funding ecosystem that places ever higher value on breakthrough science. This is problematic since, paradoxically, breakthrough potential in science builds on gradual innovation. If the value of gradual innovation is not better recognized, the potential for breakthrough innovation may well be stifled.
While continuous, gradual innovation is often taken for granted, it could become endangered in a research and funding ecosystem that places ever higher value on breakthrough science.
Concerns that the hypercompetitive dynamics of the current scientific system may impede rather than spur innovative research have been voiced for many years (Alberts et al, 2014). As performance indicators continue to play a central role for promotions and grants, researchers are under pressure to publish extensively, quickly, and preferably in high‐ranking journals (Burrows, 2012). These dynamics increase the risk of mental health issues among scientists (Jaremka et al, 2020), dis‐incentivise relevant and important work (Benedictus et al, 2016), decrease the quality of scientific papers (Sarewitz, 2016) and induce conservative and short‐term thinking rather than risk‐taking and original thinking required for scientific innovation (Alberts et al, 2014; Fochler et al, 2016). Against this background, strong incentives for fostering innovative and daring research are indispensable.  相似文献   
44.
Suggested by a previous communication on the occurence of two types of apertures among the pollen grains of Maripa s. l. (Convolvulaceae), 16 species of this genus were investigated palynologically. 3-zonocolpare grains mixed with 4–6-pantocolpate and 6–15-pantocolpate grains were found. The structure of the exine (infratectale bacula with or without supratectale processes) reveals further differences. The combination of these characters in the genus Maripa offers the separation into four types of pollen grains. The genus Mouroucoa is represented by its own type. These facts stress the necessity of the generical separation of the two genera. Consideration of pollen morphology yields some new aspects for subdivision of the genus Maripa.  相似文献   
45.
Peptide:N-glycanase (PNGase) is the deglycosylating enzyme, which releases N-linked glycan chains from N-linked glycopeptides and glycoproteins. Recent studies have revealed that the cytoplasmic PNGase is involved in the degradation of misfolded/unassembled glycoproteins. This enzyme has a Cys, His, and Asp catalytic triad, which is required for its enzymatic activity and can be inhibited by "free" N-linked glycans. These observations prompted us to investigate the possible use of haloacetamidyl derivatives of N-glycans as potent inhibitors and labeling reagents of this enzyme. Using a cytoplasmic PNGase from budding yeast (Png1), Man9GlcNAc2-iodoacetoamide was shown to be a strong inhibitor of this enzyme. The inhibition was found to be through covalent binding of the carbohydrate to a single Cys residue on Png1, and the binding was highly selective. The mutant enzyme in which Cys191 of the catalytic triad was changed to Ala did not bind to the carbohydrate probe, suggesting that the catalytic Cys is the binding site for this compound. Precise determination of the carbohydrate attachment site by mass spectrometry clearly identified Cys191 as the site of covalent attachment. Molecular modeling of N,N'-diacetylchitobiose (chitobiose) binding to the protein suggests that the carbohydrate binding site is distinct from but adjacent to that of Z-VAD-fmk, a peptide-based inhibitor of this enzyme. These results suggest that cytoplasmic PNGase has a separate binding site for chitobiose and other carbohydrates, and haloacetamide derivatives can irreversibly inhibit that catalytic Cys in a highly specific manner.  相似文献   
46.
Intravital microscopy of the murine pulmonary microcirculation.   总被引:2,自引:0,他引:2  
Intravital microscopy (IVM) is considered as the gold standard for in vivo investigations of dynamic microvascular regulation. The availability of transgenic and knockout animals has propelled the development of murine IVM models for various organs, but technical approaches to the pulmonary microcirculation are still scarce. In anesthetized and ventilated BALB/c mice, we established a microscopic access to the surface of the right upper lung lobe by surgical excision of a window of 7- to 10-mm diameter from the right thoracic wall. The window was covered by a transparent polyvinylidene membrane and sealed with alpha-cyanoacrylate. Removal of intrathoracic air via a trans-diaphragmal intrapleural catheter coupled the lung surface to the window membrane. IVM preparations were hemodynamically stable for at least 120 min, with mean arterial blood pressure above 70 mmHg, and mean arterial Po(2) and arterial Pco(2) in the range of 90-100 Torr and 30-40 Torr, respectively. Imaged lungs did not show any signs of acute lung injury or edema. Following infusion of FITC dextran, subpleural pulmonary arterioles and venules of up to 50-microm diameter and alveolar capillary networks could be visualized during successive expiratory plateau phases over a period of at least 2 h. Vasoconstrictive responses to hypoxia (11% O(2)) or infusion of the thromboxane analog U-46619 were prominent in medium-sized arterioles (30- to 50-microm diameter), minor in small arterioles <30 microm, and absent in venules. The presented IVM model may constitute a powerful new tool for investigations of pulmonary microvascular responses in mice.  相似文献   
47.
In order to study the receptor system for adrenocortical steroids in rat brain the synthetic glucocorticoid RU 28362 (11 beta, 17 beta-dihydroxy-6-methyl-17 alpha-(1-propynyl) androsta-1,4,6-trien-3-one) has been used for photoaffinity labeling. Competition and dissociation studies revealed a single class of binding sites for RU 28362 in rat brain cytosol. Photoaffinity labeling was performed by u.v.-irradiation for 2 min with a coupling efficiency of about 25%. The high efficiency permitted investigation of crude cytosolic preparations under denaturating conditions. Sodium dodecyl sulfate (SDS) and high resolution two-dimensional gel electrophoresis confirmed the high specificity of the photoaffinity labeling. The molecular weight (93 kD) as well as the isoelectric point (5.6) evaluated by these methods corresponded well to data reported for the classical glucocorticoid receptor in rat liver.  相似文献   
48.
Small interfering RNA (siRNA), double-stranded RNA (dsRNA) 21-23 nucleotides (nt) long with two nt 3' overhangs, has been shown to mediate powerful sequence-specific gene silence in mammalian cells through RNA interference (RNAi). Due to its high efficiency and high specificity siRNA has been used as a powerful post genomic tool and a potent therapeutic candidate. However, there is still a lot to learn about the mobility of siRNA inside cells and the cellular factors that might interfere with the specificity and activity of siRNA. Microglia are the brain's effector cells of the innate immune system and suitable targets in the development of novel therapeutic strategies. Here, we show the cellular uptake and intracellular distribution of siRNA in murine microglial N9 cells. siRNA was internalized by microglial N9 cells without transfection reagent and mainly localized to the endosomes However, no significant gene silencing effects were observed. Its cellular uptake and cellular distribution pattern were similar with that of a same length single stranded DNA (ssDNA). Further, cellular binding proteins of siRNA were purified and identified by mass spectrometry. Negative control siRNA and siRNA targeted to beta-actin were used in this part of experiment. Most of the siRNA binding proteins for negative control siRNA and siRNA targeted to beta-actin were dsRNA-binding proteins, such as dsRNA-dependent protein kinase R (PKR). Furthermore, both control siRNA and siRNA targeted to beta-actin activated PKR in N9 cells, which suggest that siRNA might cause off-target effects through activation of PKR.  相似文献   
49.
Zusammenfassung An zwei Stämmen Azotobacter chroococcum Beij. wurden in Montmorillonit als Bodenmodell Atmungsmessungen unter Röntgenbestrahlung durchgeführt. Durch die Bestrahlung wurde bei beiden Stämmen die Atmungsintensität erniedrigt. Aus dem Beginn der Atmungsdepression nach Einsetzen der Bestrahlung lassen sich Rückschlüsse auf die Strahlenresistenz der verschiedenen Stämme ziehen. Die Strahlenresistenz der beiden untersuchten Bakterien-Stämme war im Montmorillonit im Vergleich zu Suspensionen wesentlich erhöht.  相似文献   
50.
B. Hermann 《Genetica》1973,44(4):579-587
The chromosome complement of Meriones tristrami Thomas (Rodentia, Gerbillinae), the Israel desert jird, studied by the new technique of chromosome identification (Q and G banding) is described. The diploid number is 72. There are two pairs of submetacentric autosomes (1 and 2) and 33 pairs of acrocentric autosomes. The X chromosome is the largest submetacentric and the Y is the fourth in length among the submetacentric chromosomes of the karyotype. The Fundamental Number (F.N.) is therefore 78 and not 76 as described by Matthey in 1957.

(Travail effectué avec l'appui du Fonds de la Recherche Scientifique Médicale).  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号